Optical Multiplexer/Demultiplexer

ABSTRACT

An apparatus for optical spectrometry utilizes a simplified construction, reducing the number of independent optical elements needed while providing a sizeable dispersed spectrum. The apparatus provides a spectral intensity distribution of an input source wherein individual spectral components in the source can be measured and, in some embodiments, can be manipulated or filtered.

RELATED APPLICATIONS

This application claims rights under 35 USC §119(e) from U.S.application Ser. No. 61/113,377 filed Nov. 11, 2008, the contents ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to optical multiplexers and demultiplexers andmore particularly to a compact optical multiplexer/demultiplexer formultiplexing and demultiplexing beams of light having differentwavelengths or frequencies.

BACKGROUND OF THE INVENTION

There is a common problem in telecommunications in which optical fibershave multiple communications channels embedded in it in terms ofdifferent colors of light which are transmitting different streams ofinformation. One of the problems is to split out these channels so thatthey can be separately processed, for instance to adjust intensity,polarization and dispersion or color spread.

It is also often important in optical communications to be able tomodify each of the individual wavelengths of light differently and thenbe able to combine the processed channels so as to recombine them backinto a single fiber. Thus, it is important to break out from a singlefiber the individual spatial components, to process them and to injectthem back into a single fiber.

While the telecommunications problem described above is important, it isalso important to be able to use such a multiplexer for instance to beable to generate high energy laser beams. Presently fiber lasers existwhich can produce hundreds of watts of light within an individual glassfiber. Unfortunately, these intensity levels are not enough for somemilitary and industrial applications. The problem then becomes how to beable to utilize fiber lasers and to provide a combined output to be ableto dramatically increase the energy delivered by the system.

There is also a problem with respect to infrared laser countermeasuredevices in which laser beams modulated to countermeasure for instance anincoming missile, require a considerable amount of energy on target tobe able to robustly provide the countermeasuring function.

One type of laser used in infrared countermeasures is the so-calledquantum cascade or semiconductor laser. It is highly desirable for theseapplications to achieve higher laser powers in a low-divergence beam. Itis therefore important to be able to augment or combine semiconductorlaser outputs to provide for instance a 10 watt modulated beam ontarget.

Up to this juncture, there has been no effective way to combine theoutputs of fiber lasers or semiconductor lasers to be able tosignificantly increase the power emitted in a laser beam.

Moreover, it is important in the military context to be able to providethe power amplification modules in a sufficiently small form, to be ableto be for instance carried by a missile, carried in a DIRCM head on thebelly of an aircraft, or to provide small enough packaging to be able tobe readily used in any applications where space is at a premium.

By way of further background, optical multiplexer/demultiplexers areoptical instruments that separate out the wavelength spectral componentscontained in a single input light source. Operated in reverse, the sameinstrument combines multiple light sources of single color light into asingle output beam. In other terminology, a opticalmultiplexer/demultiplexer demultiplexes the wavelengths in the forwarddirection and multiplexes the several beams in the reverse direction. Inthe field of fiber optics communication, for example, the communicationbandwidth of a single fiber has been greatly increased using wavelengthdivision multiplexing, or WDM, techniques. Similarly, the measurementand control the properties of individual wavelengths propagating in thefiber, which is critical to the performance and operation of these WDMsystems, is performed by demultiplexing the wavelengths into individualcontrol channels.

Grating based optical multiplexer/demultiplexers are generally made upof five functional components; an input point source or linear slit, acollimating optic, the grating, an imaging optic, and one or morereceiving components in the output image. When operated as amultiplexer, the one or more receiving components are replaced bynarrowband light sources and the input source is replaced by a singlereceiving component.

Light emerging from the input source is collimated by the collimatingoptic so that a planar wavefront impinges on the (plane) grating. Thegrating breaks the single input beam up into multiple beams, with eachwavelength propagating in a unique direction. The imaging optic collectsthese diffracted beams and focuses them into spots at an output plane,where each spot corresponds to a wavelength in the source. The spotcorresponding to any single wavelength has finite size, said sizeprimarily being a function of the optical system and the grating.Operated as a multiplexer, the multiple narrowband sources (fiber laseroutputs, for example) are positioned in the “output” plane at positionsthat correspond to their central wavelengths. The imaging optic nowfunctions as a collimator, bringing the multiple wavelength collimatedbeams together on the grating, impinging on the grating at a angledetermined by the location of the source in the output plane. Thegrating redirects each beam through a unique angle, which angle ideallybrings each beam to be coaxial with all the other beams. Finally thecollimating optic brings all the parallel collimated beams into a commonfocal spot at which is located a receiving element.

One object of the present invention is to simplify the opticalconfiguration of optical multiplexer/demultiplexers by reducing thenumber of independent optical elements needed. It is another object ofthis invention to provide a physically large output spectral field whilemaintaining a compact, easy to package form factor. Yet another objectof this invention is to provide a optical multiplexer/demultiplexerusing a grating in a near-Littrow configuration.

In an alternative configuration, it is an object of this invention toprovide improved spectral resolution using only spherical reflectiveoptics.

In another configuration, it is an object of this invention to providewavelength multiplexer in which multiple independent light sources canbe combined into a single coincident output beam.

In yet another configuration it is an object of this invention toprovide a region of space in which individual spectral components fromthe source are physically accessible.

A further object of this configuration is to enable the manipulation orfiltering of individual spectral components.

Yet another object of this invention is to recombine filtered spectralcomponents back into a single beam similar in form to the source.

It is a still further object of the subject invention to provide acompact optical multiplexer/demultiplexer for use as amultiplexer/demultiplexer in a telecommunications mode and to be able tocombine laser beams of different wavelengths or frequencies to providehighly intense laser beams.

SUMMARY OF INVENTION

The present invention relates to an apparatus and method for spectrallymultiplexing and demultiplexing beams of light. More specifically theinvention relates to multiplexing or demultiplexing beams of light ofdifferent wavelengths or frequencies from multiple light sources,typically fiber lasers. This permits various applications both intelecommunications and in combining laser outputs having differentfrequencies to provide a high energy combined beam both as a weapon andfor countermeasure purposes.

In this invention, compact optical multiplexer/demultiplexer is createdusing two spherical, reflective optical elements in combination with adiffraction grating operating in a near Littrow configuration to permitlight to come in and go out from a common input direction. Thetelecentricity of the optical design in the conjugate space to the inputfiber also permits use of parallel optical fibers to input and outputlight as opposed to orienting individual fibers in different directionsdepending on wavelength or frequency.

In one embodiment, a source of light such as an optical fiber is placedat the focus of a collimator. The collimator collimates the light anddirects it to the grating. The dispersed light from the grating iscollected by the imaging optics and focused into the output plane.

The pair of reflective optical elements is used as both the collimatorand the imaging optics for the optical multiplexer/demultiplexer andresults in a compact design making the focal length minimized over asingle lens system for instance involving a 3+ foot round-trip opticalpath within a compact 6×8 inch optical footprint. One reflector is amultisection device having a parabolic collimator surface at its outeredge to collimate light towards a grating. Next to the collimator is areflective object on the multisection deflector to reflect refractedlight from the grating to an imaging optic. Light from this imagingoptic is reflected by another section of the multisection reflector toan output plane. Thus what is provided is a opticalmultiplexer/demultiplexer with only two reflective elements.

The two elements operating as the imager act as a telephoto lens, viz.,have a physical back focal distance shorter than the optical focallength. A third, refractive element, located near the source, canoptionally be added to change the input F-number and provide partialcompensation for residual spherical aberrations in the optical system.The output of the optical multiplexer/demultiplexer is a continuousdistribution of light in which the spectral distribution of energy inthe source is mapped into spatial location.

Note that although this invention has application for multiplexing aswell as demultiplexing light beams, for clarity and brevity the namesfor the optical elements throughout this specification will relate tothe demultiplexing application. The collimator is the only opticalelement between the source and the grating but the imaging opticscomprise two elements—a region of the collimator and a second reflectiveoptic.

In another configuration the optical multiplexer/demultiplexer apparatuscan be configured to multiplex multiple narrowband fiber sourcespositioned in the output plane at precise locations determined by theirwavelengths.

In a third configuration, the optical multiplexer/demultiplexer can beused to demultiplex (separate), manipulate individually, and multiplex(recombine) into one beam the various spectral components in the source.In this application the distributed image at the output of the opticalmultiplexer/demultiplexer is directed through a multi-channel filtering(or manipulating) element. The filtered light is then redirected backinto the optical multiplexer/demultiplexer, each spectral componentgenerally parallel to itself, so that the opticalmultiplexer/demultiplexer acting in reverse, now recombines thedisparate spectral components into a single beam. The single beam isformed in the input plane, displaced laterally from the input aperture.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with the Detailed Description, in conjunctionwith the Drawings, of which:

FIG. 1 shows a prior art optical multiplexer/demultiplexer in theEbert-Fastie configuration;

FIG. 2 shows a prior art optical multiplexer/demultiplexer in theCzerny-Turner configuration;

FIGS. 3A, 3B, and 3C show applications of the compact opticalmultiplexer/demultiplexer functioning either as a multiplexer for fiberlasers to provide a high power collimated beam, to combine the modulatedoutputs of semiconductor milliwatt lasers to provide a modulated highpower infrared countermeasure beam and to provide amultiplexer/demultiplexer module for correcting the individual colorchannels in a single fiber input beam to process the individual colorchannels and to output corrected color channels ported to a singleoutput fiber;

FIGS. 4A and 4B are schematic diagrams of a demultiplexer embodiment ofthe invention as laid out in the x-y plane, with 4B showing details ofthe entrance aperture region;

FIG. 5 schematically shows two means for injecting light into theoptical multiplexer/demultiplexer of FIG. 4A;

FIG. 6 is an optical design diagram of a preferred embodiment of thedemultiplexer of FIG. 4A shoving the parallel focused multispectralbeams impinging on the output plane to establish telecentricity;

FIG. 7 is the optical design diagram of an alternative embodiment of thedemultiplexer of FIG. 4A showing a segmented reflective optical elementhaving a parabolic section for collimating light onto the grating andaspheric focusing elements;

FIG. 8 is a perspective view of a preferred demultiplexer embodiment ofthe optical multiplexer/demultiplexer;

FIG. 9 is a diagrammatic illustration of the spherical surfaceconfiguration of the reflective element of FIG. 8;

FIG. 10 illustrates how the optical elements may be provided above andbelow a plane to permit light exiting or entering the device to beconveniently physically displaced so that exiting light can be processedand injected back into the subject multiplexer/demultiplexer;

FIG. 11 is a schematic view of the arrangement of optical elements topermit the operation of FIG. 10 by locating these elements in the regionaround the input aperture of the optical multiplexer/demultiplexerfunction as a demultiplexer in a plane containing the z-axis in whichthe optical elements can be configured to permit optical processing suchthat multispectral light at the output plane an be processed andinjected back into the subject optical multiplexer/demultiplexer;

FIG. 12 presents a schematic view of the subject opticalmultiplexer/demultiplexer operating as a multiplexer; and,

FIG. 13 is a schematic drawing of a multi-fiber v-block used to injectmultispectral light into the multiplexer of FIG. 12.

DETAILED DESCRIPTION

As will be discussed, a spectrometer operating at Littrow will spreadout the various wavelengths in a much shorter distance as compared withconventional non-Littrow spectrometers. While one can go far enough awayfrom a grating to establish sufficient color separation, the length ofthe device is prohibitive in many applications. Optics associated withsuch gratings are said to have a focal length dependant on colorseparation often in excess of one meter. A key advantage to the subjectmultiplexer/demultiplexer operating at Littrow is that the colorseparation is so great, the effective focal length can be reducedsubstantially to inches, making the subject multiplexer/demultiplexerdevice exceptionally compact.

Operation at Littrow refers to the directions of the collimated beams ofthe grating and the general direction of the diffracted light beingequal. To the extent that these directions are nearly the sameestablishes the greater dispersion angle and the ability to make thedevice compact.

More specifically, the importance of the Littrow configuration is thatit is the entire purpose of the diffraction grating to split colors oflight into different output angles. If all the light comes in in onedirection, and if all the colors come in collimated with a single angleof incidence, the grating will split them into a rainbow in which eachcolor comes off at a different angle. The degree of angular spreadbetween the beams coming off of the grating creates dispersion. Ingeneral, to minimize the size, and expense of opticalmultiplexer/demultiplexers, one seeks as maximize the dispersion of thediffraction grating in the utilized geometry.

If one uses a diffraction grating at Littrow with the diffracted angleequal to the indident angle, one also maximizes the dispersion of light.The spread is greatest if one is running near a Littrow configurationGenerally in order to get the beams in and out of the grating withmultiple colors one must to a certain extent move to a near-Littrowconfiguration.

Several different collimating/imaging optic configurations have beendeveloped over the years to maximize performance while optimizing otherfigures of merit. For example, the Ebert-Fastie configuration shown inFIG. 1 uses a single spherical mirror 10 as both collimator and imager,optimizing cost. Here a diverging beam through an entrance slit 12 iscollimated by mirror 10 onto grating 16. The diffracted light fromgrating 16 is focused by mirror 10 out through exit slit 14. Thisconfiguration is a non-Littrow configuration and suffers fromsignificant optical aberrations because the single optic must be usedsignificantly off-axis.

It is noted that in the prior art device of FIG. 1, a common mirror 10has a spherical surface in which light which is effectively a pointsource or a line source, is diverging and is required to be collimatedeffectively by that mirror. A spherical mirror does not collimate a beamwell. One therefore needs a parabolic mirror for a single mirror to givea very high quality collimated beam.

Referring to FIG. 2, a Czerny-Turner configuration is shown in whichspherical mirror 10 of FIG. 1 is broken up into two separate mirrors, acollimator mirror 18 and a imaging mirror 20 which again provides anon-Littrow configuration. The choice of the shape of these mirrors isoptimized so that one can optimize the collimating mirror to give a highquality collimated beam incident on grating 16. On the other handimaging mirror 20 collimates the beam from the grating out through exitslit 14, with the two mirrors being oriented in the right directions togive the best performance out of the two elements.

While there is a certain amount of performance that one can achieve outof these two elements, the optical multiplexer/demultiplexer formedthereby is exceedingly large due to the non-Littrow operation.

Note, the Czerny-Turner configuration shown in FIG. 2, uses twoindependent spherical optics for the collimator and imager. Thisconfiguration is more expensive but has lower aberrations.

Optical Multiplexer/Demultiplexer Applications

Referring now to FIGS. 3A, 3B and 3C, shown are a number of applicationsfor the subject optical multiplexer/demultiplexer operating either as anoptical multiplexer or a combined optical demultiplexer and multiplexer.

It is the purpose of the subject optical multiplexer/demultiplexer, tomultiplex or combine laser inputs of a number of colors, frequencies orwavelengths and combine them into an output beam in which the energy inthe output beam is, to a first approximation, the sum of the energies inthe input beams. What the subject optical multiplexer/demultiplexer isable to accomplish is to be able to provide high or low power collimatedoutput beams than that achievable with individual lasers that do notprovide an adequate laser power for a given application.

Nowhere is this more important than the utilization of fiber laserswhich in general are capable of high output powers of hundreds of watts,or higher. It is important to be able to combine the outputs of theselasers and sum them in a way to produce a much higher output collimatedbeam.

To this end and as shown in FIG. 3A, a number of fiber lasers 30, eachoperating at a different color or frequency have their beams 32 appliedto an optical multiplexing device 34, the purpose of which is to takethe separately colored laser beams and combine them into a collimatedhigh energy beam 36 which, is redirected for instance by a mirror 38 toan aiming head 40 which aims the beam 42 towards a target 44 to be ableto destroy the target or at least a portion thereof. The wavelengths ofthe individual lasers are chosen appropriate) to allow the opticalnultiplexing device 34 to combine them into a coboresighted output beam.

It will be appreciated that there is a limit to the power output of asingle fiber laser, and while these fiber lasers are efficient providersof collimated light, it is important to have methods to provide for thehigh intensity beams required in certain military applications.

Referring to FIG. 3B it is also possible in an infrared countermeasuresystem to provide semiconductor milliwatt lasers 50 which are modulatedby an optical modulation 52 to provide modulated differently coloredbeams 54 towards the same type of optical multiplexer that is shown byreference character 34 in FIG. 3A.

Here, the output is a modulated collimated beam 56 which is directed bya directed infrared countermeasure (DIRCM) head 58 towards a target 60,with the modulated radiation causing the seeker in the missile head ofmissile 60 to direct the missile away from its intended target asillustrated by dotted arrow 62.

The above shows a system by which lasers of different colors can havetheir outputs combined to provide a collimated beam which is highlyintense and is the sum of the output powers of the input lasers.

Referring now to FIG. 3C, the subject optical multiplexer/demultiplexermay be utilized as both a multiplexer and a demultiplexer withoutalteration of its optical components. Here an opticalmultiplexer/demultiplexer 64 is provided to take an input beam 66constituting the output from a single fiber carrying different coloredchannels.

It is the purpose of the optical demultiplexer in one mode is to couplea number of demultiplexed beams 68 to a beam processing unit 70, thepurpose of which is to alter the characteristics of the individual beamsto correct for intensity, polarization and dispersion or color spread.Such processors are known and include for instance the processorsdescribed in published US Patent Applications 2003/0067641 and2002/0176645, incorporated herein by reference. The corrected orprocessed beams 72 comprising beams of different frequencies areredirected by mirrors 74, 76, 78 and 80 back into the self-same opticaldevice, namely optical multiplexer/demultiplexer 64.

Using the same optical elements and configuration, the corrected beams,each of a different color, are combined by the multiplexer action ofmultiplexer/demultiplexer 64 such that they are recombined into a singlecollimated beam 82 which may be coupled to a single optical fiber, withthe characteristics of the individual beams corrected.

Thus as can be seen, the subject optical multiplexer/demultiplexer canbe utilized in applications where beams of multiple colors are to becombined, and wherein beams having information carried in multiplecolors may be separated out for processing, followed by multiplexingback into a single beam.

As to the optical multiplexer/demultiplexer design, and referring toFIG. 4A, a optical multiplexer/demultiplexer 80 has essentially tworeflective elements 90 and 92.

Reflective element 90 collimates the diverging light from fiber 100,with a portion 200 of reflective element 90 directing the collimatedbeam towards a grating 300.

Grating 300 disperses the incident beam and returns the diffracted beamas illustrated at 140 to a reflective surface 310 of reflector 90.

As will be described, the narrow angle α between the input and outputbeams to and from the grating is what makes the system near-Littrow,since the collimated incoming beam 130 incident in one direction isalmost in the same direction as the outgoing dispersion 140.

The diffracted energy from grating 300 is focused by a combination ofreflective surfaces 310, 320, and 330, which focuses the energy onto afocal plane 420 at focal point 410.

The result is that the overall focal length of the device itself is keptquite short as mentioned above. Also important is the fact that thesystem operates in a near Littrow configuration. Additionally, the imagespace chief rays for the various input wavelengths are parallel, acharacteristic commonly referred to as telecentricity. The opticalsystem formed by reflective elements 90 and 92 must be properly designedwith respect to the spectrally dispersed beam diffracted by grating 300to support the telecentricity condition.

In a typical nontelecentric optical system, the image space chief raysfor separate wavelength channels would not be parallel, greatlycomplicating coupling of these focused beams into individual fibers. Forexample a telecentric optical system allows the individual output fibersto be aligned in parallel in a v-groove assembly which captures all ofthe fibers in parallel tracks. In addition, parallel optical processingof the individual spectral channels by common optical processorstypically requires the individual spectral channels to be opticallyparallel.

Thus not only does the subject system separate the beams out spatially,it also produces parallel beams at the output to greatly simplifydetection and processing.

Since the system described above for the opticalmultiplexer/demultiplexer is near-Littrow, the spectral dispersion ofincoming light is magnified, thereby to provide significant spatialseparation on the focal plane for the various colors of light involved.

Demultiplexer Configuration

Referring again to FIG. 4A, this shows a schematic diagram of thecompact optical multiplexer/demultiplexer 80 in the x-y plane, i.e., theplane containing the optical axes of the various optical elements, inthe region of the entrance aperture. The z, or vertical, axis is pointedup out of the plane of the figure. When operated as a demultiplexer,light enters the instrument at the entrance aperture 100. The entranceaperture can be a physical stop or slit but, typically, the entranceaperture is simply the location in space from which the opticalmultiplexer/demultiplexer optics have been designed to accept light.Preferably, multispectral light 115 is injected at the entrance aperture100 by a cleaved or lensed optical fiber 110, wherein the core 112 ofthe fiber represents an unresolved, or point, source to the opticalmultiplexer/demultiplexer optics. Typically, the fiber core is between 5and 20 microns in diameter. In a typical optical communicationsapplication, the wavelength spectrum of the input, multispectral light115 spans one or more of the so called C or L bands (typically 1525-1565nm and 1570-610 nm respectively).

FIG. 5 shows two alternative means for injecting light 115 into opticalmultiplexer/demultiplexer 80. One alternative means uses an extrafocusing lens 111 to form the point source in the entrance aperture 100;while the second alternative uses a simple back-illuminated pinhole 114.In yet another alternative the entrance aperture is generally slitshaped, with the narrow dimension of the slit being comparable to thepinhole or fiber core 112. The long dimension of the slit is orientedparallel to the z-axis of the device illustrated in FIG. 4A.

As is well known, light emerging from the core of a cleaved opticalfiber is generally radiated into a large angled cone. Capturing all thelight in such a cone is typically difficult to accomplish. Thus, asshown in inset FIG. 4B, an interface optic 120 is preferably included inclose proximity to input fiber 110 as a means to increase the opticalefficiency of the instrument. In one embodiment, interface optic 120 isa positive power, refractive lens. If the tip of the fiber is locatedwithin one focal distance of this lens, the light emerging from the exitface of the lens will be a less broad cone. The gap between fiber 110and interface optic 120 may contain air or epoxy. For the preferredimplementation of optical multiplexer/demultiplexer 80, the source fiberradiates light 115 into a cone of numerical aperture (NA) ofapproximately 0.01 at the 1/e2 intensity and exits interface optic 120with an NA of approximately 0.022. It will be appreciated that thedetails of the optical design of this interface optic depend heavily onthe choice of input fiber, desired instrument performance and size, andso forth; and that many specific designs for this optic are possiblewithout deviating from the intent of this invention.

Returning again to FIG. 4A, the light exiting interface optic 120expands in a narrow one 118 as it propagates toward reflector 90 thatserves as primary optic 800. Primary optic 800 is a positive powerreflective optic. Preferably, for ease of manufacture, the concave,focusing surface of primary optic 800 is substantially a portion of asphere. Alternatively the focusing surface of primary optic 800 can be aportion of an asphere, as is understood in the field of optical design.The cone of light is directed such that it bypasses a secondary optic850 and impinges on a small portion 200 of primary optic 800. Portion200 functions as the collimator for the opticalmultiplexer/demultiplexer. In the preferred configuration shown in FIG.4A collimator portion 200 is located at one edge of primary optic 800.

Note, the distance between the plane input aperture 100 and primaryoptic 800, as measured along the optic's optical axis is substantiallythe effective focal length of primary optic 800. Light cone 118 is thussubstantially collimated by primary optic 800. Additionally, inputaperture 100 is located substantially on the optical axis of primaryoptic 800. However, in other configurations the entrance aperture 100can be above or below the plane of FIG. 4A. Upon reflection fromcollimator portion 200, the light propagates generally parallel to theoptical axis of primary optic 800, as collimated beam 130, until itreaches the plane diffraction grating 300. The grating is located bystandard optical ray tracing methods to intercept collimated beam 130.This position will vary relative to primary optic 800 as a function ofthe selected location for input aperture 100.

Grating 300 is selected from industry standard designs and can bedirectly ruled, replicated in epoxy, or made holographically.Preferably, grating 300 is designed to operate in the wavelength band ofinterest in the so-called Littrow configuration.

The salient characteristic of a Littrow grating, as shown in FIG. 4A, isthat the first useful diffracted order propagates generally back towardthe direction from which the input beam comes. A preferred form ofgrating 300 is the echelle grating, in which a coarse-pitched grating isused at a high grating order to achieve large angular dispersion. Thedemultiplexed outputs for all wavelengths of interest are preferablydiffracted from the grating 300 in the same grating diffraction order,with the diffraction grating grooves blazed to maximize diffractedthroughput into the utilized diffraction order. An alternativeembodiment utilizes high-order echelle gratings with each of a number ofdiscrete demultiplexed wavelengths diffracted into a differentdiffraction order, all having maximum grating efficiency for a singlegrating blaze angle.

Grating 300 diffracts beam 130 generally in the backwards direction,with each of the different wavelengths in beam 130 each being diffractedat its specific angle according to the well-known grating equation. Thecentral wavelength in beam 130, when diffracted, becomes collimatedreturn beam 140. In the preferred embodiment, grating 300 is designedaccording to well-understood principles to operate in a near-Littrowconfiguration such that collimated return beam 140 propagates back toprimary optic 800 at an angle α relative to beam 130. In the preferredembodiment, angle α is less than 10 degrees and typically equal to 4degrees. As is shown in FIG. 4A, angle α is designed to return beam 140to primary optic 800 closer to the optical axis than beam 130. That is,beam 140 is coming from larger field angle than beam 130, relative toprimary optic 800.

Beam 140 impinges on primary optic 800 in region 310, slightly displacedfrom region 200. Note, regions 200 and 310 are depicted asnon-overlapping for clarity, but may overlap by approximately 50%.

After reflecting from region 310 on primary optic 800, the diffractedlight is converted from collimated beam 140 to converging beam 150. Leftundisturbed, beam 150 would come to a focus in essentially the sameplane as input aperture 100, but displaced downward in FIG. 4A by theeffect of angle α. It will be understood by one skilled in the art thatbeam 150, while referred to in the singular when it represents the beamformed by a single wavelength from the original spectrum of the inputlight, the single wavelength to be substantially at the center of theoperating wavelength band of optical multiplexer/demultiplexer 80, isactually is meant to indicate the continuum of beams propagating in theoptical multiplexer/demultiplexer, each wavelength in the source havingbeen uniquely directed by the dispersive nature of grating 300. Eachadditional wavelength in the source has its own converging cone leavingregion 310, with each cone coming to focus in a common output plane 420,thereby forming a dispersed spectrum in that plane.

In optical multiplexer/demultiplexer 80, the one or more beams 150 donot propagate directly to focus. Instead, they are reflected fromsecondary optic 850 and again by primary optic 800. The converging beamsimpinge on secondary optic 850 in a small region near its edge,indicated by region 320, and then impinge on primary optic 800 in region330. Together the three regions 310, 320, and 330 form a long focallength telephoto imaging system that focuses the collimated beam(s) fromthe grating into a continuous intensity distribution 410 at output plane420. This represents the spectral content of input light 115. Thus, theinvention has demultiplexed a multiple wavelength input beam. In thepreferred implementation, the imaging system has an effective focallength of 180 mm and forms an f/23 cone at the distribution 410.

When the optical multiplexer/demultiplexer is used as a demultiplexer,the imaging system formed by primary optic 800 and secondary optic 850forms a magnified image 410 of the spectrum on the sensitive surface ofa detector 400, which has been located in the output plane 420. Thedetector is preferably selected to respond to the wavelength band ofinterest. Typically, detector 400 is an array detector composed ofmultiple independently readable detector elements, although film andspectroscopic plates may also be used. Preferably, the magnification ofthe imaging system is designed such that the element-to-element spacingin the preferred detector produces the desired spectral sampling. Arefractive field lens may be added in proximity to the detector plane toflatten the image field and if desired, to adjust the telecentricity ofthe spectrograph.

In an alternative configuration, a relay magnification optical systemcan be located after image 410 to adjust the size of image 410 to matchthe desired spectral sampling with the element-to-element spacing in thepreferred detector. The use of such an image magnification-matchingrelay is well known in the art.

Returning to FIG. 4A, the preferred optical design for the telephotoimaging system comprising regions 310, 320, and 330 has a back focallength (essentially the distance at which the spectrum 410 is formed) of17 mm. Since the back focal length is shorter than the effective focallength (180 mm), the imaging system is a telephoto design, providinghigh magnification in a compact package.

Functionally, the collimator and imaging system optics for thisdemultiplexer are formed from three independent elements correspondingto regions 200/310, 320, and 330. These three elements, each of which isan eccentric pupil subaperture of a larger parent optical element, mustbe mounted and aligned relative to each other to form the collimator andimaging systems described above. Preferably, the collimator and imagingsystem are assembled from the two full-width elements themselves, i.e.primary optic 800 and secondary optic 850. Each of these parent opticsis a section of a centered spherical mirror with a diameter determinedby the off-axis locations of regions 200 and 320 (for optic 800 and 850respectively). The manner in which these elements are sectioned aredescribed below. Since these two optical elements are centered, they canbe easily mounted and aligned using simple techniques that arewell-known in the industry and will be described below as well.

The optical design parameters for the preferred embodiment of themultiplexer/demultiplexer have been determined using commercial opticaldesign software. Table I is the optical design diagram showing thepreferred embodiment with the corresponding optical design listing (“thelens prescriptions”). It will be noted by those of skill in the opticaldesign art that the entire system comprises two powered elements, thesebeing reflective spherical optical elements.

TABLE 1 Lens Prescription for 1547 nm Wavelength, NA 0.02 input RDY THIOBJ: INFINITY 3.457700 STO: INFINITY 0.000000  2: INFINITY 0.000000  3:INFINITY 118.654508 XDE: 0.000000 YDE: −69.149831 ZDE: 0.000000 REV ADE:11.514052 BDE: 0.000000 CDE: 0.000000  4: −439.64270 −123.285456 REFLSLB: “M1”  5: −251.80151 123.285456 REFL SLB: “M2” XDE: 0.000000 YDE:7.425031 ZDE: 0.000000 DAR ADE: −4.676107 BDE: 0.000000 CDE: 0.000000 6: −439.64270 0.000000 REFL SLB: “M3”  7: INFINITY 0.000000 XDE:0.000000 YDE: −36.860825 ZDE: 0.000000 REV ADE: 4.012577 BDE: 0.000000CDE: 0.000000  8: INFINITY −169.251359 XDE: 0.000000 YDE: 36.770465 ZDE:0.000000 REV ADE: −4.012577 BDE: 0.000000 CDE: 0.000000  9: INFINITY166.672006 REFL SLB: “GRT” GRT: GRO: −22.000000 GRS: 0.018986 GRX:0.000000 GRY: 1.000000 GRZ: 0.000000 XDE: 0.000000 YDE: 48.643024 ZDE:0.000000 DAR ADE: 63.577583 BDE: 0.000000 CDE: 0.000000 10: −439.64270−217.650880 REFL SLB: “COL” IMG: INFINITY 0.000000 XDE: 0.000000 YDE:13.729200 ZDE: −0.000000 DAR ADE: 11.828300 BDE: 0.000000 CDE: 0.000000

As mentioned above and referring now to FIG. 6, it is a feature of thesubject invention that the optics involved provide a telescope havingtelecentricity. As can be seen, spherical reflector 90 collimates lightfrom a point source onto grating 300 which diffracts the multispectrallight as ullustrated at 802 back towards spherical reflector 90. Fromthere a portion of spherical reflector 90 focuses the incident lightonto a reflective element 320 which in turn continues the focusingaction and returns the light to another portion of spherical reflector90, with each of the different colored beams spatially separated.

These spatially separated beams on spherical reflector 90 are againfocused to output plane 420 such that individual beams of light 808 ofdifferent colors impinge on output plane 420 from the same direction.The arrival of these differently colored beams of light in parallelpermit detection by detectors 400 or coupling of the light intorespective optical fibers positioned at plane 420 such that thecenterlines of these fibers are also parallel one to the other and cantherefore line up with the corresponding beams. This alignment is madesimple due to the parallelism of beams 808 which can be matched to theparallelism of the optical fibers. This permits the use of the v-channelpositioning block of FIG. 13.

Referring to FIG. 7, in certain circumstances a higher performancesystem may be desired. In such circumstance the design performancelimitations of a two element pure spherical system may be improved byremoving one or both of those constraints (viz., two element constraintand spherical only constraint). FIG. 7 is the optical design diagram foran embodiment freed from those constraints in which the individualoptical surfaces may be a mixture of spherical, conic, or asphericsurfaces, here respectively a parabolic section 810, an aspheric section812, an aspheric surface 814 for element 850, and an aspheric section816. What is shown here is a segmented mirror configuration for primaryoptic 800.

As shown in FIG. 8, all components of optical multiplexer/demultiplexer80 are assembled as a compact package, mounted on a unitary basestructure 1000 for stability. Each component is preferably mounted usingpre-determined reference points, typically miniature dowel pins.Analyses have shown that system performance will not be degraded whensaid alignment reference points are positioned to within 0.001-0.002inches, a tolerance well within standard practice in optical assemblytechnology. Preferably the structure is athermalized. For example, basestructure 1000 is preferably manufactured from Zerodur®, a well knownsubstrate material, although fused silica and ULE® glass are acceptablealternatives. Additionally, kinematic or quasi-kinematic mountingconfigurations are preferred.

Referring to FIG. 9, a perspective view of the instrument shows thatprimary optic 800 and secondary optic 850 preferably implemented asrectangular sections 801 cut from parent optical elements 802 and 804.The thickness, T, of the elements is equal to the clear aperturerequired for beam 118 when it reaches primary optic 800 plus additionalmargin for manufacturing, alignment and mounting considerations, as istypically done in optics manufacture. Alternative manufacturingapproaches, such as, optical replication, injection molding, or diamondturning are anticipated by the inventors.

Spectral Manipulator Configuration

Another application for the invention is to permit individual and uniquemanipulation, or filtering, of the various spectral components that makeup the light emitted from the source. For example, one might want toequalize the optical energy across the visible spectrum in the lightemitted by a blackbody source of known emission temperature. A secondpreferred configuration of the invention is used for this application.For filtering, the optical multiplexer/demultiplexer is used in a doublepass mode. That is, the light is injected into the opticalmultiplexer/demultiplexer in the input plane, is dispersed passingthrough the instrument, and is formed into a continuous spectrallydispersed display in output plane 420 as before. However, detector 400is replaced by other elements that firstly manipulate each component ofthe spectrum individually and secondly return the light into the opticalmultiplexer/demultiplexer instrument for a second pass. During thisreverse, second pass, the previously spectrally dispersed light isrecombined by the aforementioned grating element into a single beam andis refocused into an output point by the opticalmultiplexer/demultiplexer optics.

The modifications to the invention to accommodate this double passoperation are described with reference to FIGS. 10 and 11.

In an optical multiplexer/demultiplexer configuration, it is desirableto use a single optical multiplexer/demultiplexer 80 to spatiallydemultiplex the spectral components or channels of an input signal tiersome form of optical processing, and to subsequently multiplex theprocessed light signals back into a single output beam.

FIG. 11 presents a geometry for the demultiplexed region after theoptics in which multiple sequential Spectral Manipulating Elements(SMEs) can be inserted and the light retroreflected appropriately suchthat the optical multiplexer/demultiplexer 80 will properly multiplexthe signals. The retroreflection path illustrated in FIG. 11, whencombined with the optical multiplexer/demultiplexer 80 illustrated inFIG. 4A, directs the multiplexed processed output out of themultiplexer/demultiplexer.

As shown in FIG. 10, the optical multiplexer/demultiplexer 80 of FIG. 4Amay be modified to direct the output beam through an exit aperture 105and output fiber 110 a spatially separated from the entrance aperture100. FIG. 10 is a view of optical multiplexer/demultiplexer 80 in theplane of cut A-A′ in FIG. 4A in the region of the entrance and exitaperture. In this second preferred configuration, entrance aperture 100is offset out of the plane of 106 FIG. 4A below plane 106. That is, theaperture is displaced in the z-axis direction below plane 106. Exitaperture 105 exists above plane 106 to capture the processed beam. Note,the entrance aperture is shown offset in the negative z-direction (i.e.,below the plane of FIG. 4A) although a positive offset is equallypreferred. The offset entrance aperture 100 is matched symmetrically byan exit aperture 105 positioned above plane 106, in the z-direction.Multispectral light is introduced into optical multiplexer/demultiplexer80 as in the prior configuration, shown in FIG. 4A, as expanding cone118. The propagation of this light through the instrument proceeds as inthe prior configuration and exits optical multiplexer/demultiplexer 80as beam 118 a.

FIG. 11 shows the additional elements in the region near output plane420 required to implement the processing of the dispersed beams from theoptical multiplexer/demultiplexer and to allow the spatially separatedinput and output apertures of FIG. 10. Detector 400 of FIG. 4A isreplaced by the combination of one or more spectral manipulationelements (SME) 430, 432, and two broad band reflective surfaces 440,442. Preferably, surfaces 440, 442 are high reflection optical coatingsdeposited on optical substrates (viz., front surface mirrors) as is wellknown in the art, where said coatings have been optimized for reflectingthe operational wavelength band of the invention.

As will be appreciated. FIG. 11 is a view of opticalmultiplexer/demultiplexer 80 in the region of cut B-B′ in FIG. 4A.Reflective surfaces 440 and 442 are positioned such that the normals totheir respective surfaces are each in the plane formed by the centralray of beam 150 and the z-axis. Additionally, the reflective surfaces440, 442 are held at a right angle to each other. As is well known inthe field of optics, surfaces held in this orientation retroreflectlight beams in the plane in which the mirrors are folded. For beamspropagating with a component out of aforementioned plane, the in-planecomponent is retroreflected and the out-of-plane component will bereflected as if the mirror pair were a single mirror (i.e., the angle ofreflection equals the angle of incidence).

As shown, first reflective surface 440 is located in front of imageplane 420 by displacement distance, D, 153 and oriented at substantially45 degrees to the direction of propagation of beam 150, with itsreflective side facing beam 150. First reflective surface 440 isgenerally located such that the central ray of beam 150 interceptssurface 440 substantially at its center. Second reflective surface 442is located at substantially 90 degrees to first surface 440 and alsofacing the beam 150 and located such that the central ray of beam 150intercepts its surface substantially at its center. Displacementdistance D is substantially equal to one-half the distance betweensurface 440 and surface 442, as measured along the central ray of beam150. In this configuration, output plane 420 a, the plane containing theoptical axes of the optical components, is substantially parallel tobeam 150 and located midway between surfaces 440 and 442.

Preferably, light converging toward output plane 420 first passesthrough SME 430, whose function will be described below. Note that beam150 is traveling above (z-positive) the plane 420 a containing theoptical axes of the optical components. The beam is above the axis nearoutput plane 420 because input aperture 100 is located below said plane,as was described above. Similarly, beams 150 would be below this planehad the input aperture been place above the plane.

Beam 150 impinges on surface 440 and is reflected downwards (e.g.,generally along a direction parallel to the z-axis) toward surface 442.Output plane 420 a is formed mid-way between surfaces 440 and 442, inaccordance with the said selection of displacement distance 153. Theforward pass of this double pass configuration is complete when beam 150reaches its focused condition in output plane 420 a, wherein acontinuous spectral distribution is formed.

As the light passes through focus in output plane 420 a it re-expands indefocusing beams 150 a as it begins its second pass through the opticalmultiplexer/demultiplexer 80. The spectral distribution in output plane420 a now serves as the source for the opticalmultiplexer/demultiplexer. Emerging from output plane 420 a (i.e. plane106 of FIG. 10) beams 150 a impinge on reflective surface 442 and arereflected into a plane substantially parallel to, but displaced in thenegative z-direction from the plane containing beam(s) 150, with thesaid displacement being substantially equal to the aforementionedpositive displacement of beams 150. The redirected beams are pointedgenerally back to primary optic 800 and appear to be emerging from aspectral distribution in output plane 420 c. The beams diverging fromoutput plane 420 c toward primary optic 800 preferably pass firstthrough SME 432, whose function will be described below.

Beams 150 a propagate generally back to primary optic 800 until theyreach region 330. From region 330 backwards-propagating beams 150 agenerally retrace the paths taken by forward-propagating beams 150.Beams 150 a are generally slightly displaced from their correspondingbeams in beams 150, the displacement being in accordance with wellunderstood optical ray tracing analyses and having no significance tothe effect of the various optics in optical multiplexer/demultiplexer80.

When beams 150 a reach region 310 they become substantially collimatedbeams 140 a. Beams 140 a are in direct correspondence with forwardpropagating beams 140. Beams 140 a impinge on grating 300 at angles ofincidence corresponding to their wavelengths and, thus, are rediffractedby grating 300 into a common propagating direction where they arecollectively considered a single beam, beam 130 a. Beam 130 a propagatesto region 200 of primary optic 800 where it is focused toward exitaperture 105 as beam 118 a. At the exit aperture, the focused beam 118 ais typically coupled into an optical fiber and out of the opticalmultiplexer/demultiplexer 80.

The function of the double pass configuration just discussed is toprovide a physical space in which one or more spectral manipulationdevices can operate on the various spectral components in the originaloptical signal independently. That is, if the optical signal travelingin the input optical fiber is composed of light at two distinctwavelengths, say 1540 nm and 1550 nm, it is the purpose of thisinvention to allow one optical adjustment to be applied to the 1540 mmlight and a separate and independent adjustment to be applied to the1550 nm light. For example, it may be desirable to attenuate the 1550 nmlight by 10% without affecting the 1540 nm light in order to equalizethe optical power in the two wavelengths.

In FIG. 11, the preferred configuration, in which two SMEs areinstalled, is illustrated. Alternatively, a single SME could be usedalone, positioned in the location of either SME 430 or 432 or in theoutput plane 420 located between the reflective elements 440,442. Afurther alternative configuration would permit three SME's to be used,with one SME in plane 420 and one each corresponding to SME 430 and 432.

Preferably, the SME's are located close to output plane 420, said planebeing the location at which the various spectral components are mostdistinct. However, exact positioning in this plane is not required sincethe spectral manipulation performed by the SME's are always of finitespectral resolution themselves and, typically, are slowly varying withwavelength. The manipulation performed by a SME 430 may be continuouswith spatial position (said spatial position corresponding to differentwavelengths) or spatially discrete. A physically large example of theformer is a variable neutral density filter, such as Newport Researchmodel 50G02AV.2 while an example of the latter could be a DichroicFilter Array as produced by Ocean Optics Inc. of Dunedin, Fla. usingtechnology under license of U.S. Pat. No. 5,711,889, Methodology to makeoptical filter arrays. Any of a large variety of SMEs may be used in theinvention, and different numbers and types of SMEs may be mixed andmatched in the invention while remaining within the intent of invention,which is to make a physically accessible region available in whichindividualized manipulation of the spectral components is possible.

Multiplexer Configuration

Referring to FIG. 12, the multiplexer configuration is shown, here athird configuration for optical multiplexer/demultiplexer 80 is toreverse the functions of the input and output planes to create a meansof multiplexing several individual light sources into a single, combinedbeam. The optical system for the multiplexer configuration, shown inFIG. 12, except for the elements in the input and output planes, isunchanged from the demultiplexer configuration shown in FIG. 4A. Formeroutput plane 420 is, in this configuration, the location for one or morelight sources, typically narrowband fiber lasers 510, held in a fiberpositioning block 500. Each laser 510 is tuned to a unique wavelengthwithin the dispersive range of grating 300. The source may alternativelybe a semiconductor laser array with individual laser facets tuned to theappropriate laser wavelength. Former input plane 100, typically, is justa reference location through with the multiplexed beam will pass atfocus.

For one preferred embodiment, grating 300 is operates at the 22^(nd)order in a near-Littrow configuration and, with the 2 mirror opticaldesign, the angular dispersion of the grating is converted into aphysical dispersion of 100 GHz/millimeter at output plane 420. Outputplane 420 is approximately 50 millimeters wide, so the multiplexingrange of this embodiment is approximately 5000 GHz.

In a multiplexer, it is desirable to precisely overlap each of the inputbeams to obtain high output beam quality. To achieve said precisionoverlap, each input source is positioned precisely in former outputplane 420. In a preferred embodiment as shown in FIG. 13, each source isan optical fiber 510 mounted in a v-channel 520 in positioning block500, with the location of the v-channel for each fiber having beencalculated specifically for the wavelength to which the source is tunedand the specific grating 300 and optical elements in the system.

Each fiber in positioning block 500 emits a cone of light 150. It willbe understood that only one cone is illustrated for clarity but inoperation there will be multiple, nearly parallel cones of light beingemitted simultaneously. As illustrated in FIG. 12, the light in cone 150propagates “backwards” through the optical multiplexer/demultiplexer,reflecting sequentially from region 330, region 320, region 310, grating300, and region 200 before coming to focus in plane 100. At former inputaperture 100 all of the cones of light that are emitted from block 500are substantially overlapped to form a single output beam. Analternative embodiment takes advantage of the fact that the input laserbeams are overlapped and coboresighted into a high quality collimatedbeam after diffracting from grating 300. This beam may be used as thefree-space collimated output of the multiplexer if the grating 300 isoriented at an appropriate angle α such that the output beam clearsoptical element 800.

A preferred form of grating 300 is the echelle grating, in which acoarse-pitched grating is used at a high grating order to achieve largeangular dispersion. The multiplexed outputs for all wavelengths ofinterest are preferably diffracted by the grating 300 into the samegrating diffraction order, with the diffraction grating grooves blazedto maximize diffracted throughput into the utilized diffraction order.An alternative embodiment utilizes high-order echelle gratings with eachof a number of discrete multiplexed wavelengths diffracted into adifferent diffraction order, all having maximum grating efficiency for asingle grating blaze angle. Accordingly, in this alternative embodiment,all multiplexed wavelengths emerge into a common angular direction; eachwavelength being diffracted at or near the peak efficiency of the blazefunction for that discrete wavelength and diffracted order. The peak ofthe blaze function repeats with diffracted order and in this embodiment,each wavelength uses a different diffracted order. So, wavelengthseparations can be chosen such as to align with the peak diffractionefficiency of subsequent diffracted orders. For example, an echellegrating can be created such that the order separation corresponds toapproximately 100 GHz in frequency or 0.8 nm in wavelength in the 1.5micrometer wavelength region. Such a grating would provide peakdiffraction efficiency at each discrete wavelength in the diffractionorder in which it is used, sending all wavelengths into a common outputangular direction. Another embodiment of grating 300 would be amultilayer dielectric interference grating, allowing high power laseroutput.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

1. An optical multiplexer/demultiplexer comprising: two reflectiveoptical elements in combination with a diffraction grating operating ina near Littrow configuration to concentrate in a demultiplexingoperation a diverging incoming multispectral beam and collimate ittowards said grating and to function as a telescope to take thediffracted light from said grating and focus it towards an output planesuch that beams of different colored light arrive in parallel at saidoutput plane to establish telecentricity, or in a multiplexing operationto take differently colored light beams arriving at said output plane,focus them onto said grating and collimate the refracted light from saidgrating into a single multispectral output beam.
 2. The apparatus ofclaim 1, wherein the near Littrow operation is established byconfiguring said two reflective optical elements such that the directionof the collimated multispectral beam direction is nearly identical tothe direction of said diffracted light dispersed across a set ofdirections.
 3. The apparatus of claim 1, wherein the pair of reflectiveoptical elements serve as both the collimator and imaging optics for themultiplexer/demultiplexer.
 4. The apparatus of claim 1, wherein saidreflective optical elements have spherical reflective surfaces.
 5. Theapparatus of claim 1, wherein said optical elements have sphericalreflective surfaces.
 6. The apparatus of claim 5, wherein one of saidpair of reflective optical elements is segmented, with one segmenthaving a parabolic surface and wherein other sections have asphericreflective surfaces.
 7. The apparatus of claim 6, wherein said parabolicreflective surface is at the periphery of said segmented reflectiveoptical element.
 8. The apparatus of claim 7, wherein one section ofsaid segmented optical element reflects refracted light from the gratingto the said imaging optic.
 9. The apparatus of claim 8, wherein lightfrom said imaging optic is reflected from another section of saidsegmented optical element to said output plane.
 10. The apparatus ofclaim 1, wherein said optical elements operate as an imager and as atelephoto lens having a physical back focal distance shorter than thefocal length.
 11. The apparatus of claim 1, and further including anoptical processor at said output plane.
 12. The apparatus of claim 11,wherein said processor individually processes multispectral light beamsand injects processed muitispectral light beams back into saidapparatus, whereby said apparatus functions as a multiplexer to combinesaid processed beams into a single multispectral beam.
 13. The apparatusof claim 1, and further including multiple differently colored laserbeams projected through said apparatus to be combined into an outputbeam having power which is the sum of the power of the input laserbeams.
 14. The apparatus of claim 13, wherein each of said laser beamsare modulated, whereby the combined output beam includes thecorresponding modulation.
 15. The apparatus of claim 14, wherein saidmodulation is used in a countermeasure application.
 16. A compactoptical multiplexer/demultiplexer, comprising: a grating and a twoelement optical system in which the focal length of the optical systemis minimized over a single lens system such that the round trip opticalpath is short to provide a compact optical footprint, said opticalsystem operating near Littrow and exhibiting telecentricity, saidoptical design providing that light comes in and goes out from a commoninput direction.
 17. The apparatus of claim 16, wherein the opticalfootprint of said compact design is on the order of 6×8 inches.
 18. Theapparatus of claim 16, wherein said apparatus includes two opticalelements, one of said optical elements providing a collimating functionat one reflective surface thereof, with both of said optical elementsproviding a telescope focusing function that provides saidtelecentricity.
 19. The apparatus of claim 18, wherein the lightreflected from one of said optical elements collimates light onto saidgrating and wherein diffracted light exits in nearly the same directionas the collimated light, thus to establish the Littrow condition. 20.The apparatus of claim 18, wherein said opticalmultiplexer/demultiplexer has an optical centerline and wherein saidgrating is to one side of said centerline.
 21. A compact apparatus foroptical spectrometry comprising: a first reflective element having anoptical axis and separate collimating region and converging regions,said collimating region receiving input optical radiation andcollimating said input optical radiation into a collimated beam; and, adiffraction grating displaced from said optical axis for receiving saidcollimating beam and dispersing said collimated beam into a plurality ofdispersed beams which are provided to said converging region forconvergence into a converged plurality of beams.
 22. The apparatus ofclaim 21 further comprising: an imaging system for receiving saidconverged plurality of beams and providing an intensity distributioncorresponding to the spectral content of the input optical radiation.23. The apparatus of claim 22 further comprising: a detector formeasuring the intensity distribution.
 24. The apparatus of claim 21,wherein said input optical radiation has a wavelength spectrum spanning1525-1565 nm.
 25. The apparatus of claim 21, wherein said input opticalradiation has a wavelength spectrum spanning 1575-1610 nm.
 26. Theapparatus of claim 21, wherein said input optical radiation is receivedfrom an optical fiber.
 27. The apparatus of claim 26 further comprising:an entrance aperture transmitting said input optical radiation.
 28. Theapparatus of claim 27, wherein said entrance aperture is substantially apinhole.
 29. The apparatus of claim 27, wherein said entrance apertureis substantially a slit.
 30. The apparatus of claim 29, wherein thenarrow dimension of said slit is comparable to the narrow dimension ofthe optical fiber.
 31. The apparatus of claim 27 further comprising: afocusing lens interposed between the source of the input opticalradiation and said entrance aperture.
 32. The apparatus of claim 27further comprising: an interface optic interposed between the source ofthe input optical radiation and said entrance aperture.
 33. Theapparatus of claim 32, wherein said interface optic is a positive power,refractive lens.
 34. The apparatus of claim 32, wherein input opticalradiation enters said interface optic with a numerical aperture ofapproximately 0.01 at 1/e² intensity and exits said interface optic witha numerical aperture of approximately 0.022.
 35. The apparatus of claim27, wherein the distance between said entrance aperture and said firstreflective element is substantially the effective focal length of saidfirst reflective element.
 36. The apparatus of claim 21, wherein saidfirst reflective element is a positive power reflective optic.
 37. Theapparatus of claim 21, wherein said first reflective element issubstantially a portion of a sphere.
 38. The apparatus of claim 21,wherein said first reflective element has a reflective surface that issubstantially an asphere.
 39. The apparatus of claim 21, wherein saidcollimating region is small relative to the size of said firstreflective element.
 40. The apparatus of claim 21, wherein saidcollimating region is sited at an edge of said first reflective element.41. The apparatus of claim 21, wherein said diffraction grating is oneof a directly ruled grating, an epoxy-replicated grating, and aholographically-replicated grating.
 42. The apparatus of claim 21,wherein said diffraction grating is placed in a substantially-Littrowconfiguration.
 43. The apparatus of claim 21, wherein said collimatingregion and said converging region are non-overlapping regions on saidfirst reflective element.
 44. The apparatus of claim 21, wherein saidcollimating region and said converging region are overlapping regions onsaid first reflective element.
 45. The apparatus of claim 22, whereinsaid imaging system comprises: a second reflective element for receivingsaid converged plurality of beams and providing a reflected convergedplurality of beams; an imaging region on said first reflective elementfor receiving said reflected converged plurality of beams and providingsaid intensity distribution.
 46. The apparatus of claim 23, wherein saiddetector is selected to respond to light in a predetermined spectralrange.
 47. The apparatus of claim 23, wherein said detector is one of afilm, a spectroscopic plate, and a detector array composed of multipleindependently readable detector elements.
 48. The apparatus of claim 23further comprising: a refractive lens in proximity to said detector. 49.The apparatus of claim 23 further comprising: a relay magnificationoptical system in proximity to said detector.
 50. The apparatus of claim21, wherein the back focal length between said converging region andsaid detector is shorter than the effective focal length of theapparatus.
 51. The apparatus of claim 21, wherein said first reflectiveelement is a rectangular element.
 52. The apparatus of claim 25, whereinsaid second reflective element is a rectangular element.
 53. Theapparatus of claim 21 further comprising: a unitary base structure forholding said first reflective element and said diffraction grating in afixed configuration.
 54. The apparatus of claim 53, wherein said unitarybase structure is made of a low expansion material.
 55. The apparatus ofclaim 22 further comprising: an entrance aperture displaced verticallyrelative to the apparatus; and an exit aperture displaced verticallyrelative to the apparatus and opposite said entrance aperture.
 56. Theapparatus of claim 55 further comprising: a first spectral manipulationdevice for receiving said intensity distribution and providing a firstfiltered intensity distribution; and, a reflective element for providingsaid first filtered intensity distribution as input to the imagingsystem.